In order to increase efficiency, a high turbine inlet temperature is used in standard gas turbines. As a result, there can arise high NOx emission levels and higher life cycle costs. These can be mitigated with a sequential combustion cycle, wherein the compressor delivers an increased pressure ratio. A main flow passes a first combustion chamber (for example, using a known burner of the type as disclosed in EP 1 257 809 or as in U.S. Pat. No. 4,932,861, also called an EV combustor, where the EV stands for environmental), wherein a part of the fuel is combusted. After expanding at a high-pressure turbine stage, the remaining fuel is added and combusted (for example, using a burner of the type as disclosed in U.S. Pat. No. 5,431,018 or U.S. Pat. No. 5,626,017 or in U.S. Patent Application Publication No. 2002/0187448, also called SEV combustor, where the S stands for sequential). Both combustors contain premixing burners, to lower NOx emissions.
Because the second combustor is fed by expanded exhaust gas of the first combustor, the operating conditions can allow self ignition (spontaneous ignition) of the fuel air mixture without additional energy being supplied to the mixture. To prevent ignition of the fuel air mixture in the mixing region, the residence time therein should not exceed the auto ignition delay time. This criterion can allow flame-free zones inside the burner but can pose challenges in obtaining appropriate distribution of the fuel across the burner exit area.
SEV-burners can be designed for operation on natural gas and oil. Therefore, the momentum flux of the fuel can be adjusted relative to the momentum flux of the main flow so as to penetrate in to the vortices. The subsequent mixing of the fuel and the oxidizer at an exit of the mixing zone can be just sufficient to allow relatively low NOx emissions (mixing quality) and avoid flashback (residence time), which may be caused by auto ignition of the fuel air mixture in the mixing zone.